The pole windings of a generator rotor need to form a complete circuit and pole crossovers are used to this effect. Though pole crossovers can take the form of many different shapes and sizes, effectual pole crossovers for generator rotors are thick circular or semicircular bands of strong, highly conductive metal such as copper.
FIG. 1 illustrates one type of semi-circular pole crossover. Axial arms 2 connect to either pole of the generator rotor winding and are joined together by a semi-circular band 4. Though not shown, the band abuts rotor windings on its outer circumference. The axial arms 2 proceed inboard into slots in the rotor forging body. The portions of the rotor winding outside of the rotor forging body are referred to as the rotor end windings. The axial arms 2 and band are joined, in this example, at square corners 6, which are either contiguous or a brazed joints. During operation, the rotor is rotated at high speeds. The rotor winding is not rigid enough to provide for its own centripetal acceleration which maintains its radial location, therefore a heavy ring of steel, known as a retaining ring, is used to retain the end winding during operation while wedges contain the straight copper in the rotor forging body slots. During operation, the retaining ring deflects some amount in the radially outward direction. Due to the summation between the tangential momentum of the copper and the centripetal acceleration permitted by the retaining ring, the rotor end winding, including the pole crossover, conforms to this deflection of the ring. Since the axial arms 2 are rigidly supported in their slots in the rotor forging body, the radial deflection 8 effectively enlarges the diameter of the rotor end windings, including the pole crossover. This creates a difference of movement between the band 4 section of the pole crossover and the axial arms 2, causing deflection related stresses at the square corner 6.
These stresses on the square corner create metal fatigue which can progress into cracking and eventually failure of the crossover, which in turn will cause a loss of the generator electrical field. Many of these generators start up and shut down on daily cycles, providing for relatively rapid accumulation of metal fatigue damage and driving crack propagation. To replace a failing crossover is a time consuming process, which requires that the generator be off-line for extended periods and which risks damage to other parts of the generator such as the winding insulation.
What is needed is a method and apparatus that accommodates the deflection related stresses to which the pole crossover is subjected. Furthermore, a method and apparatus is needed that can accomplish this with minimal invasion to the generator system as well as having the ability to be readily retrofitted into existing machines.